Chromium metallurgy



Patented Dec. 31, 1940 UNITED STATES CHROMIUM METALLURGY Marvin J. Udy,Niagara Falls, N. Y.

No Drawing. Application September 24, 1937,

Serial No. 165,588

9 Claims. (CI. 75-52) This invention or discovery relates to-chromiummetallurgy, and it comprises a new intermediate material useful inchromium metallurgy and having a composition generally like that of achromium garnet, or uvarovite, but containing a substantially lessamount of silica and more chromium, said material being substantiallyfree of carbon, iron oxide and of other foreign metallic oxides; and itfurther comprises a process 10 of obtaining chromium from materialscontaining both chromium and iron wherein chromium metal is selectivelyoxidized in the presence of lime and suflicient silica to form a calciumsilicochromite at a melting temperature and unoxidized iron metal isseparated as a byproduct,

the silicochromite being an advantageous material for making chromiummetal or, when mixed with a non-carbonaceous reducing agent such assilicon or a silicon alloy, for adding to molten ferrous metals to formchromium alloy steel; all as more fully hereinafter set forth and asclaimed.

Metallic chromium-has become of great importance in the manufacture ofnon-corrodible or oxidationresistant metals. It is usually used in'theform of iron chromium alloys such as standard ferrochromium containingat least 66" per cent chromium, or a 2:1 ratio of chromium to iron. Formost uses the ferrochromium must be nearly carbon free and this involvesgreat expense in manufacture. In obtaining ferrochromium for use inmaking stainless steel and like alloys, the world is combed for highgrade chromite ores, the mineral ferrous chromite being substantiallythe only source of chromium now known. Some of these ores are poor,containing much gangue and little reducible chromium, and most of themare low-grade in the sense that the ratio of iron to chromium is high.

' 40 Many ores are both poor and low-grade.

In making ferrochromium by reduction of chromite ores, it has beenconsidered possible to use only native high chromium ores in which aportion of the FeO of the spinel mineral,

' FeCr204, is replaced by, another'oxide such as of chromite by limeor'similar base. The natural ore is smelted in an electric furnace withlime and a limited amount of reducing agent, the lime replacing the ironoxide in the chromite and the replaced iron oxide being reduced to metaland separated from an ore melt containing most of the chromic oxide ofthe chromite ore combined with lime as a chromite, together with thegangue of the ore.

In said prior application is disclosed and 10 claimed a specificmodification of said beneficiating method wherein a portion of naturalore is treated with a ferrochromium, the metallicreduction products ofchromite ore, in the presence of lime. The selective oxidation of the 15chromium in the ferrochromium used produces an enrichment of the oretreated.

In this modification the ore is beneficiated ,both by replacement. andremoval of iron and by addition of chromic oxide. In said process, iron20 ore can be used instead of chromite ore for the purpose ofselectively oxidizing the chromium of ferrochromium in the presence oflime forming a calcium chromite ore. These modifications of theinvention of my prior acknowledged 25 application are described andclaimedin a. copending application Serial No. 164,988, filed September21, 1937 (Patent No. 2,127,074).

In the present invention, I selectively oxidize the chromium containedin ferrochromium or 30 4 in other metals and alloys in the presence oflime and silica and produce iron free compounds of lime, chromic oxideand silica, which are here designated silicochromites, and a byproductmetal which, if desired, may be made extremely 5 low in chromium. Theiron and chromium in a a metal containing both are separated byselective oxidation at a melting temperature in the presence of lime (orsimilar base) combining with the chromium oxide formed, suflicientsilica be- 40 ing added to the reaction mass to,form readily fusiblecalcium silicochromites which have a lower fusing point than that of apure calcium chromite. These silicochromite compounds have a compositionsimilar to that of 45 the relatively rare mineral uvarovite or chromiumgarnet," a mineral belonging to the garnet class. In nature it is notfound pure or of ideal composition, there being usually A120: replacingpart of its CraOa. It is a refractory material. When of idealcomposition, it is Ca:Crz(SiO4) s. The silicochromites formed in thepresent invention are less rich in silica than the ideal min eral andare free of alumina, iron, etc. They are rich in easily reduced chromiumand par- SI ticularly adapted for making chromium and its compounds.Ordinarily I make products containing not more than two-thirds as muchsilica as uvarovite and I may go down to a silica content of a third ofthat in the ideal mineral. As I have found, these calcium silicochromitecompounds are formed in the molten state from ferrochromium attemperatures suitable for the selective oxidation of chromium metal andare readily separated from the molten iron metal remaining unoxidized.However, it is possible to use either more or less silica, and toseparate molten compounds approaching the highly siliceous idealuvarovite in composition on the one hand and on the other a calciumchromite, CaCrzO4 combined with one molecule or less of silica. The lesssilica in the compound, the higher is its melting point. While it ispossible in an open arc furnace to make a calcium chromite free ofsilica, it is advantageous to add silica in moderate amount and tap oifa molten silicochromite. Uvarovite .of theoretical composition isrelatively low in chromium and high in silica content. When the chromiumis reduced,

'much slag is formed.

Calcium chromite has an extremely high melting point when not containingresidual ferrous chromite and its direct production in the electricfurnace by total reduction and removal of the iron of chromite orerequires inconveniently high temperature in the electric furnace. In thepresence of a little silica, more convenient temperatures arepracticable. Enough silica to approach uvarovite in composition isunnecessary and gives too much slag in later operations.

I have found these artificial silicochromites, which are substantiallyfree of carbon and iron, to be of great utility for making chromiummetal by reduction thereof with silicon and for making alloy steel byreduction of the CrzOa content with ferrosilicon serving as reducingagent. For example, a material corresponding approximately to theformula 3CaO.Cr2Oa.SiO2 is particularly useful by reason of itssubstantial chromic oxide content which is about 40 per cent and itsready fusibility at moderate electric furnace temperatures and in theopen-hearth steel furnace.

Chromium alloy steels of the lower chromium contents are made in a basicopen-hearth furnace and it is advantageous to have a source material forchromium fitting in with the requirements of an ordinary basicopen-hearth furnace. This the present material does. It adds no aluminaor other foreign oxides to the ordinary floating slag of calcium-ironsilicate. The heat of reaction in reduction. of the CrzOa by silicon iscarried directly into the steel bath, making it possible to produce inthe open hearth furnace steels of higher chromium content thanheretofore.

For use in making alloy steel, the silicochromite in granulated orfinely ground form, is advantageously mixed or briquetted withfine-ground ferrosilicon and added to the steel bath in a relativequantity giving the desired chromium content in the finished steel. Theiron of the ferrosilicon joins the bath and the silica from the siliconjoins the slag. The chromite can be made as low in iron as desired andwhen mixed with, say 50 per cent ferrosilicon, which is readily madeextremely low in carbon, not only has the advantages of a carbon-freeferro-chromium but also that of the exothermic reaction setting freechromium and adding heat to the bath: 2CrzOa+3Si=4Cr+3Si02+l00,200calories.

The addition of a mixture of silicochromite and ferrosilicon to a steelmelt has no appreciable chilling effect upon the heat.

A selective or differential oxidation of chromium in a ferro alloy canbe effected according to the following reaction,

(1) Fe+2Cr+CaO+30=Ca0 C12O3+Fe in which oxidation of the Cr may beeffected by the oxygen ofthe air or other source such as pure oxygen orby iron oxide or iron ores or other metallic oxides which are readilyreduced. There is considerable heat evolved in this reaction and ina-properly designed furnace, very little power is required to keep uptemperature after the oxidation begins until near the end when the CH0;has reached 40 per cent or more when power is necessary to complete thereaction.

The selective oxidation process is advantageously carried on in anelectric furnace but may be carried out in a blast furnace or a bessemerconverter.

In order to prevent oxidation of iron it is necessary to operate in anatmosphere non-oxidizing to iron. In the electric furnace I have foundthat oxidation of the chromium is very rapid and will take place ifthere be sufficient coke on the bathto prevent oxidation of the iron.Sufficient coke is carried on the bath to maintain a CO concentrationpreventing oxidation of the iron.

It is advantageous to use iron oxide in iron ores or chrome ores forselective oxidation of chromium as described in my Patent 2,098,176, andI have found in making high grade calcium silicochromite great advantagein the exothermic action of F6203 in oxidizing chromium metal. In

large heats this method is quite successful and is an important part ofthe process. In carrying out the process exothermetically it is usual touse sufiicient ferrosilicon or silicon metal and an oxidizer such assodium nitrate to give additional heat and greater fluidity;

I have further found that chromium at high temperatures is oxidized byC02 and this oxidation is much more vigorous than the correspondingreaction between CO2 and Fe. In the case of chromium the reaction doesnot appear to be reversible. o The reaction for iron is given below (2)and the corresponding reaction for chromium (3).

Reaction 3 may be slightly reversible theoretically but from myexperience it does not appear to be so to any material extent. It canreadily be made to go to completion.

One method in which I make use of this retion is to smelt in an electricfurnace ferrochrome of any Cr to Fe ratio or chromium bearing irons oralloy steels or chromium bearing scrap with calcium carbonate (limerock) and silica to form a compound approximating 3CaOCI2O3S1O2 (or Cokeis added to the bath in suflicient quantity to maintain a COconcentration in the CO: atmosphere preventing oxidation of the Feduring and after oxidation of the chromium. I have found that suflicientcoke for this purpose can be carried on the bath, without affectingoxidation of the chromium.

The final products of this process whetherI begin with scrap metalcontaining chromium or with ferrochrome made from ores of any Cr to Feratio are iron having a low but substantial content of chromium andcalcium silicochromites.

Refractory materials such as A120 and MgO are substantially absent incontrast to natural chromite ore. The calcium silicochromite may bereadily converted to chromium compounds such as chromic acid, sulphate,oxide, etc. by known methods.

The following example illustrates the process and results obtained inmaking the new silicochromite product. In this case the melting wascarried out in an open are electric furnace of the Heroult type.

The metal used analyzed .42 per cent Cr, 6 per cent C, 3 per cent Si, 49per cent Fe. This metal to the amount of 2380* pounds was mixed with3050 pounds of lime stone (53 per cent CaO) and 350 pounds of quartzite(96 per cent S102) and melted in the electric furnace using coke on thebath to insure sufiicient CO concentration to prevent oxidation of theFe.

The calcium silicochromite produced was approximately 3564 poundsanalyzing Cr2O3 38.5

per cent; CaO 47.4 per cent; SiOz 14 per cent.

As byproduct 1197 pounds of molten metal containing 8 'per cent Or wereseparated. The chromium in the byproduct metal can be controlled toalmost any desired content. Usually I have made a sufiicient number ofheats to give a sizable amount of metal to cast in moulds and thencarried the oxidation to reduce the Cr in the metal under 3 per cent.Economic conditions usually determine how far the oxidation should becarried.

The following example shows a specific embodiment of my inventionincluding the production of a finished alloy steel containingapproximately 18 per cent chromium. The silicochromite produced as inthe preceding example is ground and briquetted with ferrosiliconcontaining 50 per cent silicon, which has a low carbon content, insufficient quantity to reduce the Cl'zOs present in the silicochromite.Thus I use 100 parts of silicochromite with 25.5 parts of 50 per centferrosilicon (10 per cent excess) and add as briquettes or as fine mixto a previously prepared steel bath containing 108.5 parts of low carbonsteel. After the reaction and refining period there are produced 145parts of steel containing 17.3 per cent chromium and less than .10 percent carbon. The carbon content depends on the reduction obtained in theiron preparation previous to the addition of the silicochromite andferrosilicon briquettes. The slag produced the slag as SiOz.

is 95.2 parts containing approximately 1.00 per cent CH: and has alime-silica ratio of about The carbon free silicochromite used in theabove example is iron freeand the cost of production is higher than whensome iron is oxidized during the chromite formation. The iron freeproduct however is highly useful in making high grade chromium metal forspecific uses such as in the manufacture of alloys of the stellite andnichrome types.

Following is a specific example of my invention in the production ofhigh grade chromium metal; One hundred pounds of the silicochromite ofthe above composition are mixed with 13.3 parts per cent excess) of 97to 98 per cent silicon metal and smelted in an open arc furnace theHeroult type. The silicon reduces the Orzo: to chromium metal and the Sienters The slag is adjusted to a limesilica ratio of about 1.5 CaO to 1SiOz by the additon of 12.7.. parts of CaO. The metal produced was 24.6parts or about 90 per cent recovery and analyzed upwards of 98 per centCr. The

slag was approximately 104.5 parts and analyzed 2.4 per cent Crzoa.

It will be recognized that, when this product of silicochromite is usedin making alloy steels, an iron free product is not required. Indeed itis much cheaper to make a product containing some iron as FeO. Theprocess can be and is operated to produce such a product. The followingare typical analyses of successive heats made to produce asilicochromite with some iron present but of a Cr/Fe ratio high enoughto produce the standard 70 per cent ferrochrome,.or to be used toincrease the ratio of Cr to Fe in ores having a Cr to Fe ratio too lowto produce a '70 per cent ferrochrome or higher.

' Heat N 0.

CH0 FeO Gr/Fc ratio One skilled in the art can readily recognize thevarious possible uses for such products.

Another use of the silicochromite made as described is in the productionof chromates, bichromates and chromic acid. The Cr203 of silicochromiteis readily oxidized to C10 by roasting in the air at temperatures of-700to 1000 0. Calcium chromate is formed 'and is readily leached from theinsoluble calcium silicate. Chromic acid is advantageously produced byacidifying the calcium chromate in solution with ,sulfuric acid,precipitating calcium sulfate.

chromium in a molten condition with lime and silica and an oxidizingagent selectively reacting upon the chromium to form a fusible calciumsilicochromite containing the greater part of the chromium of theferrochromium as oxide and from 14 to 28 per cent silica and separatingfrom said chromite a molten metal containing the greater part of theiron of the ferrochromium.

3. A process which comprises treating ferrochromium in a moltencondition with lime and silica and iron oxide selectively reacting uponthe chromium to form a fusible calcium silicochromite containing thegreater part of the chromium of the ferrochromium as oxide and from 14to 28 per cent silica and separating from said chromite a molten metalcontaining the greater part of the iron of the ferrochromium.

4. A process which comprises treating ferrochromium in a moltencondition with calcium carbonate and silica selectively reacting uponthe chromium to form a fusible calcium silicochromite in pure formcontaining the greater part of the chromium of the ferrochromium asoxide and from 14 to 28 per cent silica and separating from saidchromite a molten metal containing the greater part of the iron of theferro-chromium.

5. A process which comprises selectively oxiidizing the chromium offerrochromium in the presence of silica and a base and at a meltingtemperature to obtain a basic silicochromite in a molten massseparatedfrom a molten metal containing most of the iron of/ theferrochromium, mixing said silicochromite with ferrosilicon and addingthe mixture 'to a molten steel bath.

6. A process which comprises selectively'oxi- Patent No. 2,227,287.

MARVIN J. may.

cEnrmcArr: F, comcrromf dizing. the chromium of ferrochromium in thepresence of silica and a base and at a meltin temperature to obtain abasic silicochromite in a molten mass separated from a molten metalcontaining .most of the iron of the ferrochromium and reducing saidsilicochromite with silicon to obtain metallic chromium.

'7. A process which comprises selectively oxidizing the chromium offerrochromium in the presence of silica and a base and at a meltingtemperature to obtain a basic silicochromite in a molten mass separatedfrom a molten metal containing most of the iron of the ferrochromium andreducing said silicochromite with a silicon alloy.

8. In a process-for producing a product containing oxidized chromiuminvolving the treatment of a molten alloy containing iron and chro-MARVIN J. UDY.

December 51, 191 0.

It is hereby certified that-error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,-sec- 0nd column, line "58, for "exothermetically" read-:--exothermical1y--; line 58-59, for "retien" read "reaction" and line72, for "exothermeti'cally" read --exothemically--; page 5, firstcolumn, .line 14., before "forms" insert fi'rst--- dter "furnace" insert--of--" line 5,-for "Ca read "60 second column, line 18,

, and that the-said Potters Patent should be read with this correctiontherein that the same may conform to the'reeord of the case in thePetentoffice.

signed and sealed this 17th day of March,

(Seal) Henry-Yen Arsdale, Acting-commissioner of Patents.

CERTIFICATE OF CORBECTION.

Patent No. 2,227,287. December 31, 19h0.

MARVIN J. UDY.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,second column, line "58, for "exothemetically" read-.--exothermically--;line 58-59Qfor "ration" read -reaction--; and line 72, for"exothermeigically" read -exothensically-; page 5, first cOlumn,.1 ineh, before Proms" insert -first-; line 5, for Ca read -C0 second column,line 18,

after "furnace" insert -of--; and that the said I etters Patent shouldbe read with this correction therein that the same may conform tothe-record of the case in the Patent Office.

Signed and sealed this 17th day or March, A. n. 1912.-

. flenry van Arsdale, (seal) Acting commissioner of Patents.

